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The rear wheel rim on my mountain bike cracked the other day, due likely to wear from braking over a period of a few years while using v brakes. It got me to thinking is there a way to determine when a rim is worn out? There seemed to be no obvious sign of excessive wear on my bad rim, yet it cracked. I do not remember hitting any unusual obstacles, most of my riding has been on roads.

I don't know about mountain bikes rims, but most road bike rims with aluminum braking surfaces have a wear indicator built into the rim. If the rim is worn past that indicator, then the rim needs to be replaced. I'm sure they have mountain bike rims with wear indicators, but I could be wrong.

older rims and even some new ones sometimes don't have a wear indicator.

rims with external wear indicators, the cheap ones have a small groove on the outside. once that groove is gone, the rim needs to be replaced.
rims with internal wear indicators will develop holes once the rim has worn past a certain amount. these can be found on many higher quality rims like open pro.

so, how to judge... the braking surfaces will become concave were the pads hit most often.
it's a bit hard to judge unless you are really staring at it when it's clean.
obviously the rim will wear out faster with harder pads and riding conditions which kick up a lot of debris like sand, salts and mud.